Electrodeposition of metals using pyrophosphates



United States Patent 3,039,942 ELECTRODEPOSITIDN 0F METALS USING PYROPHOSPHATES George Chandler Cox, 3711 Washington Ave., and Walter Elwood Vail, 1541 Bridge Road, both of Charleston 4, W. Va. No Drawing. Filed Sept. 20, 1961, Ser. No. 139,375 7'Ciaims. (Cl.20438) This invention relates to improved methods and processes for electrolytically depositing metals withoutthe necessity of immersing the object to be electroplated in an aqueous solution containing a soluble compound of the metal to be deposited, whereby the cost of electrolyte is greatly reduced and the quality of the deposit and the throwing power are improved.

Other than as disclosed in the Cox application Serial No. 690,640, now US. Patent No. 2,919,233, as far as is known'nomethod has been developed for electroplatinga conducting object or structure except when the object is immersed in or is in contact with a solution of a compound of the metal-Which it is desired to deposit. In the above-mentioned patent methods have been described whereby the amphoteric metals lead, tin and zinc may be electrolytically deposited on a cathodic metal structure coated with a composition containing a relatively insoluble compound of the metal to be deposited when the structure to be plated is immersed in an electrolyte containing a salt of an alkali metal but containing no soluble compound of the metal to be deposited.

An object of the present invention is to reduce the corrosion of the inner surfaces of tank ship compartments, of large oil and chemical storage tanks, of large chemical apparatus and of other large corroding surfaces by the deposition of a protective metal on such surfaces where the cost of using the usual electroplating methods would be prohibitive.

Another object is to accomplish the electrolytic deposition of certain metals, either singly or as alloys, from coatings of i their relatively insoluble compounds on a cathodic structure immersed in or in contact. with an electrolyte containing no soluble compound of the metal or metals .to be deposited but containing one or more soluble compounds that will incrementally solubilize the metal components of the coating by formation of'complex'ions. Complexing compounds which may be used in electrodepositingmetals by this process include ammonia, the ammonium salts, the pyrophosphates, the cyanides, the fluoborates, the fluosilicates, the thiosulfates, or a salt of an'organic acid suchas a citrate, oxalate or tartrate. Metals which we have electrodeposited from coatings containing their relatively insoluble compounds Whentheelectrolyte has contained one or more ofthe aforementioned complexing compounds include: copper, silver, zinc, cadmium, tin, lead, cobalt, and nickel. These metals have been deposited through the employment of the combined action of one ormoreof the-aforementioned complexing agents and the'hydroxyl ion, the complexing effect of the latter having been claimed in US. Patent No. 2,919,233.

Another. object ism-electrolytically deposit copper, silver, zinc, cadmium, tin, lead, cobalt or nickel from a composition which has been applied as an adherent coating to a conducting structure and which contains compounds of low solubility of the specific metal or metals to be deposited, and'frorn. which theaforesaid specific metal content is incrementally solubilized and cathodically deposited on said conducting structure during passage of current through an electrolyte that contains 'a'pyrophosphate of an alkali metal.

Another object is'to electrolytically deposit one or more metals from the group consisting of copper, silver,

3,39,942 Patented June 19, 1952 zinc, cadmium, tin, lead, cobalt and nickel from a composition which has been applied as an adherent coating posited on said conducting structure during passage of current through an electrolyte that contains two or more complexing agents.

From the foregoing objects it is apparent that these procedures consist of two distinct and essential parts: (I) the methods of forming on an electrically conducting structure a suitable adherent coating of low solubility containing a compound of the metal to be electrolytically deposited when the structure is subsequently madecathodic, and (II) the methods of incrementally solubilizing such a coating at the cathode interface so that the desired metal can be electrolytically deposited.

For purposes of simplifying the language, the following definitions will be used throughout this application:

An adherent coating will be defined as an adherent, non-metallic coating which can be formed by any of the various methods herein discussed on an electrically conducting structure, which is a coating containing one or more compounds of low solubility of the metal to be deposited and which is permeable to the electrolyte.

It is intended that the conducting structure to be electroplated be defined as including any clean electrically conducting structure, either metallic or non-metallic. Also, it is intended that ferrous metal structure be defined as including the various steels and any other alloyof iron which has or has not been electroplated with one or more other metals, or otherwise surface coated with a metal as by dipping, cladding, and the like.

I. Methods of Forming an Adherent Coating An adherent coating may be formed on a conducting structure in a number of Ways, four of which are illustrated in the following examples, the choice of method depending on the nature of the conducting structure and the metal to be deposited: V

(a) As a first step in preparing such an adherent coating on an electrically conducting structure, the structure may be coated with a basic coating of low solubility which does not contain the metal to be deposited in accordance with the teachings of US. Patents No. 2,200,469 and No. 2,5 34,234. For this step the forming electrolyte would be either sea water or a solution containing magnesium ions. Because of the desirability of a porous deposit for the present invention the need for calcium ions for densification as discussed in these patents is not a requirement. Also, in operation under the teaching of these patents use of the high magnesium field for increased porosity of the basic deposit is desirable. As described inlater paragraphs, the initial basic coating of low solubility may then be treated by dipping, spraying or-otherwise wetting the coating under controlled conditions with a soluble salt of the metal to be deposited, such as the chloride, nitrate, sulphate, etc. The controlled conditions would include such items as concentration of metal ion, concentration of hydrogen ion, time and temperature. As a result of these steps an adherent coating containing a compound of low solubility of the desired metal can be formed.

If it is desired to deposit lead when using the procedure described in paragraph (a) above, the soluble lead salt may-be either lead "nitrate or lead acetate. For example, when a small panel coated with an initial basic coating of low solubility was treated for one hour at room temperature with a normal solution of lead nitrate having a pH value of 3.8 the weight gain was 0.35 gram. No appreciable further gain in weight was obtained When the treating time was extended up to 48 hours, and for treat ing times materially less than one hour the weight gain dropped ofi rather rapidly. Higher temperature treatments resulted in a material reduction in the treating time. for the maximum weight gain. This eifect of time and temperature in general was found for other metals also. For example, a maximum weight gain was obtained on a coated panel when it was treated for five minutes at 75 12 C. with a normal solution of zinc chloride having a pH of 5.5. A maximum weight gain with zinc sulphate was likewise obtained under these conditions. When panels coated with an initial basic coating of low solubility were treated at 83:2 C(for two hours in a normal solution of nickelous chloride having a pH of 6.4 useful weight gains were obtained.

a When normal stannous chloride solutions were used at room temperatures for treatment of the initial basic coating described in paragraph 1(a), the low pH of approximately 1.0 resulting from hydrolysis of the chloride caused solution of the initial basic coating in about 15 seconds. However, when tenth normal stannous chloride solutions having pH values of approximately 2.1 were used at room temperature fairly good adherent coatings containing tin were obtained with treatments of about two minutes. Illustrating the effect of higher pH values, when similarly coated panels were treated at room temperature (19 C.) for six hours in a normal solution of cadmium chloride having a pH value of useful weight gains were obtained. For any particular metal the optimum combination of temperature, time, acidity and metal ion concentration can readily be determined. In general, the pH of the metal salt soltuion should be adjusted to incipient precipitation. V

It should be emphasized that room temperature conditions are highly desirable for low cost treatment of large tanks and structures, but it is explicitly understood that the examples given in this application do not constitute a limitation in any way whatever on any of the various reaction conditions and variables involved in the procedures herein described.

V (b) If it is not convenient to give the structure an initial basic coating of low solubility in accordance with the teachings of the above-mentioned patents, a composition for such a coating may be conveniently made by mixing in proper proportions magnesium hydroxide and magnesium chloride to give a composition related to the well known magnesium-oxychlon'de cements. Then a su tably cleaned conducting structure may be coated with a slurry of this composition by any of the standard procedures such as by spraying, dipping or painting and allowing it to harden into an adherent coating which does not as yet contain the metal to be electrodeposited. Other chemical compositions for the initial coating may similarly be used, provided they are somewhat basic, such as a calcium cement. Then a desired metal content may be chemically precipitated by Wetting the coated surface of the structure with a solution containing a metal salt under controlled conditions as discussed above, thereby forming the desired adherent coating.

(c) A hydroxide or other compound of low solubility of a metal to be deposited may be made into a cement by the use of suitable inorganic or organic bonding agents. When it is desired to electrodeposit zinc on a conducting structure, the structure may be coated with a slurry of a freshly made zinc oxychloride or phosphate cement. For example, a slurry of 40% ZnO, 12% ZnCland 48% water by weight was pasted on copper panels and allowed to harden overnight and electrolyzed as described in section II into a good deposit of zinc. When lead is the desired metal to be deposited a suitably thin slurry of litharge and glycerin may be applied to form an adherent coating by any of the usual methods (spraying, dipping or painting) and then allowed to harden. For example, a slurry of 78% litharge and 22% glycerin byweight was pasted in a thin film onto mild steel panels and allowed to harden overnight before being electrolyzed as discussed in section II.

(d) An adherent coating of low solubility containing any of the metals of this invention may be formed on a conducting structure by adding the necessary amount of a.

.very finely divided relatively insoluble compound of the desired metal to a magnesium-oxychloride cement slurry, and then applying this slurry as a thin coating over the surface to be electroplated and allowing it to harden. For example, a slurry of 27% MgO, 45% MgCl .6H O, 10% NiO and 18% water was applied as a slurry, allowed to harden overnight, and electrolyzed as described in section II to give a deposit of nickel. Other cements or suitable inorganic or organic bonding agents may be used as binders, provided that no shrinkage cracks occur in such a cement coating when it has set, and provided that it is permeable to the electrolyte subsequently used in plating.

II. Methods of Solubilizing the Metal Content of an Adherent Coating One of the essential features of this invention is that the metal content of an adherent coating is incrementally solubilized at a very slow rate at the cathode-coating interface, thereby providing a low concentration of the required metal ions from which the desired metal, or metals, is subsequently deposited electrolytically. For, in accordance with present electroplating theory, a requirement for producing dense, fine-grain deposits of low porosity is that the metal ion concentration must be maintained at all times. during electrolysis at a very low value in comparison to the total available metal content which in this invention is to be electroplated from the adherent coating composition.

Oneprocedure which fulfills these conditions has been disclosed and claimed in US. Patent No. 2,919,233. What we now propose and claim in this application, in addition to some methods of forming adherent coatings not disclosed in US. Patent No. 2,919,233 are other procedures for incrementally solubilizing the metal content of an adherent coating by the formation of complex ions.

The following examples will illustrate various methods of solubilizing the metal compound of an adherent coating by the formation of one or more complex ions, and electrolytically depositing the desired metal from these ions. We have obtained good metal deposits by the use of not only complex cations but also of complex anions. For example, we have incorporated in the adherent coating a hydroxide or other relatively insoluble compound of the metal to be deposited and progressively solubilized it at the interface in the form of complex anions by the use of an electrolyte containing an alkali metal pyrophosphate. Examples of such soluble complex pyrophosphate anions containing the metal are:

It is generally recognized that the solubilizing action of the pyrophosphate ion is due to the combination of one or more of these ions with an insolublevmetal pyrophosphate molecule to form a soluble complex anion. As an illustration, when using sodium pyrophosphate as a solubilizing agent, these reactions can be expressed as follows:

[P207] l-Zl'lg (P207) (insoluble) a [Zn P207) E (soluble) and it is this latter ion from which, either directly or in directly, the metal is deposited on the cathode.

With the amphoteric metals, lead, tin and zinc, this effect is in addition to the complexing effect of the hy- The soluble plumbite ion, [Pb(OH) l and stannite ion, [Sn(OH) are similarly formed. (See General Chemistry, by Linus Pauling, 2d ed. (1959), page 481.)

Based on the results we have obtained in experimental work When plating zinc, tin or lead by the method herein claimed and using sodium pyrophosphate as the complexing agent, it appears that the above-mentioned pyrophosphate complexing action is the controlling factor in providing improved plating over and above that obtained when'the solubilizing eflect is only that of forming the zincate, stannite or plumbite complexes. For example, tests have shown that when one uses a sodium pyrophosphate electrolyte a given weight of plate of any one of these three metals can be obtained in one-half to twothirds the time required when only a sodium chloride electrolyte is used with the same current density and same sodium content,

Further supporting evidence of this double-barreled complexing action is indicated by the fact that for a given current density and given time, say 24 hours, a heavier deposit of zinc, tin or lead can be obtained when one uses an alkali pyrophosphate than can be obtained when only an alkali chloride or sulphate is used. Also, the current efiiciency is greater when an alkali pyrophosphate is used rather than an alkali chloride or sulphate and a distinct improvement in plating uniformity is obtained.

This indicates that the pyrophosphate complexing action is the controlling factor when alkali pyrophosphates are used even though some zincate, stannite or plumbite ion is presumably formed also.

The following examples illustrate this type of reaction: When an adherent coating containing lead was converted at 75 :1 C. as described in foregoing paragraph 1(a) and then immersed at room temperature in an M/8 solution of sodium pyrophosphate and made cathodic at 50 ma./ sq. ft. for 48 hours a bright, silvery, fine-grain, well bonded and very uniform deposit of lead was obtained. With all other conditions similar, except that a sodium chloride electrolyte of equivalent sodium content was used, the resulting deposit was approximately 60 percent of the weight of lead deposited with the abovementioned pyrophosphate electrolyte. In other words, the pyrophosphate bath gives a higher current efiiciency than that obtained when only the hydroxide complexing action of an alkali salt is used.

When an adherent coating containing zinc was formed on steel panels as given in paragraph 1(a) above, then immersed in an M/ 8 solution of sodium pyrophosphate and made cathodic at 100 ma./sq. ft. for 48 hours a bright, silvery, fine-grain, well bonded and very uniform deposit of zinc was obtained. When other conditions were the same except that a sodium chloride electrolyte having a sodium content double that of the sodium pyrophosphate bath was used, the resulting deposit weighed slightly less than 70 percent of the zinc deposited from the above-mentioned sodium pyrophosphate bath. This is a further indication that the combined complexing action of the pyrophosphate and hydroxide ions results in a materially higher current efliciency and a somewhat superior grade of electroplate.

Further, when steel panels covered with adherent coatings containing tin were cathodically reduced at 100 ma./sq. ft. for 24 hours in a normal solution of sodium pyrophosphate and in a normal solution of sodium chloride a similar improvement in the current efliciency of the panels that were reduced in the pyrophosphate bath was noted.

In regard to some of the non-amphoteric metals covered in this application the following examples are typical of the results obtained. In a sodium pyrophosphate bath,

we have electroplated silver on a graphite electrode from an adherent coating. This adherent coating was formed as outlined in paragraph 1(a) by treating on initial basic coating for one hour at room temperature in an N/ 1, silver nitrate solution of which the pH had not been adjusted. The coating was then electrolyzed at 150' ma./ sq. ft. for 24 hours at room temperature in a solution which was N/ 1 with respect to the sodium pyrophosphate, whereby a thin fairly uniform deposit of silver was plated out on the graphite cathode.

When copper panels covered with cobalt containing adherent coatings formed as in paragraph 1(a) were cathodically reduced at ma./sq. ft. for 24 hours in a normal solution'of sodium pyrophosphate good deposits were obtained. An excellent electroplate of cadmium copper formed over the graphite electrode.

General These procedures allow a conducting structure to be electroplated with any metal which can be placed on the structure as an adherent coating of controlled thickness and then incrementally solubilized through the formation of complex ions and cathodically deposited from the solubilized product. As previously indicated, we believe one one explantion of the fine grain and low porosity of deposits which we have obtained by the use of complex ions for solubilizing the metal content of an adherent coating is that they provide an extremely small concentration of metal ions for electrolytic reduction at the cathode as has been found desirable in use of these complex ions in ordinary electroplating baths. However, the increase in throwing power and covering power which can be obtained in the electrodeposition of metals by our procedures is probably due largely to the uniform distribution or" the adherent coating over the entire surface being electroplated. This results in a more uniform distribution of the metal to be deposited than is possible where metal ions depletion or a decrease in current density at a remote or shielded point will cause serious variations in metal deposition. Another probable reason for the improved results which are characteristic of these processes is the increased resistance and consequent voltage drop across the adherent coating, which in effect amounts to a greater cathode polarization than that ordinarily obtained in the customary plating bath. This increase in elfective polarization would be expected to cause a greater uniformity of deposition, a decrease in crystal size of the metal deposit, a decrease in treeing, and an increase in throwing power.

Within reasonably wide limits of cathodic current density, from 10 ma./sq. ft. to 12.5 amp/sq. ft., and with tenth normal up to normal solutions of one or more compounds that will cause complex ion formation under the conditions herein described, we have obtained firmly bonded electrolytic deposits of the desired metal on a cathode, although the best deposits formed by any of the methods herein disclosed were made at low current density with low electrolyte concentrations. Higher temperatures of the reduction baths generally allow the use of higher current densities.

We have obtained excellent deposits at current densities between 50 and 200 Ina/sq. ft. and have observed no appreciable falling off in quality of deposit at current densities as low as 10 ma./sq. ft. The maximum current density should not be so high that the metal deposit will be burned, spongy, non-uniform or arboreal in structure.

I The optimum current density will depend on such factors as temperature, resistance across the adherent coating and on the nature of the cathode, of the metal being deposited, and of the electrolyte.

The concentration of the complexing compound in the electrolyte must not be so great as to dissolve the adherent coating rapidly and allow it to diffuse away from the cathode before there has been time for the metal ion to be deposited electrolytically thereon. This precaution is more necessary with those electrolytes which produce complex anions containing the metal thanvwith those which form the complex cations. When the electrolyte contains an ammonium salt, the major solubilizing efiect occurs only with the flow of current, which liberates ammonia at the cathode with subsequent formation of soluble metal amines. For production of plate of high quality the optimum concentration of complexing compound in the electrolyte will depend on the nature of the metal beig deposited, on the nature of the cathode surface, and on such variables as current density, temperature, pH and the amount of non-complexing salt that may also be present in the electrolyte.

While the preferred embodiments of the invention are herein disclosed by way of example, it is understood that the invention is'not limited with respect to the precise modes of forming an initial basic coating of low solubility, to the modes of forming an adherent coating of low solubility containing the metal to be deposited, or to the modes of solubilizing and electrodepositing the metal content of the adherent coating, within the spirit of the appended claims.

This application is a continuation-in-part of copending application Serial No. 842,021, filed October 1, 1959.

We claim:

1. A process of electrolytically depositing on a conducting structure a metal selected from the group consisting of copper, silver, zinc, cadmium, tin, lead, cobalt and nickel, which comprises applying to said structure an adherent coating containing a compound of low solubility of the metal to 'be deposited, and then subjecting said coated structure to a cathodic electromotive force in an aqueous electrolyte containing an alkali metal pyrophosphate which, through the combined action of the pyrophosphate ion and the hydroxyl ion liberated at the cathode, will incrementally solubilize the metal component of the coating as complex ions at the cathode while at the same time the selected metal is electrodeposited on the cathode from the solubilized products.

2. The method or" claim 1 wherein the conducting structure is a metal structure.

3. The method of claim 1 wherein the conducting structure is a ferrous metal structure.

4. A process of electrolytically depositing on a conducting structure a metal selected from the group consisting of copper, silver, zinc, cadmium, tin, lead, cobalt and nickel, which comprises applying to said structure an adherent coating containing a compound of low solubility of the metal to be deposited, and then subjecting said coated structure to a cathodic electromotive force in an aqueous electrolyte containing an alkali metal pyrophosphate which will incrementally solubilize the metal component of the coating as a pyrophosphate complex at the cathode while at the same time the selected metal is electrodeposited on the cathode from the solubilized product.

5. The method of claim 4 wherein the conducting structure is a metal structure.

6. The method of claim 4 wherein the conducting structure is a ferrous metal structure.

7. A process of electrolytically depositing lead on a ferrous metal structure which comprises applying to said structure an adherent coating containing a compound of lead of low solubility and then subjecting said coated structure to a cathodic electromotive force in an aqueous electrolyte containing an alkali metal pyrophosphate which will slowly solubilize the lead component of the coating as a soluble pyrophosphate complex ion at the cathode while at the same time lead is electrodeposited on the cathode from the solubilized product.

No references cited. 

1. A PROCESS OF ELECTROLYTICALLY DEPOSITING ON A CONDUCTING STRUCTURE A METAL SELECTED FROM THE GROUP CONSISTING OF COPPER, SILVER, ZINC, CADMIUM, TIN, LEAD, CABALT AND NICKEL, WHICH COMPRISES APPLYING TO SAID STRUCTURE AN ADHERENT COATING CONTAINING A COMPOUND OF LOW SOLUBILITY OF THE METAL TO BE DEPOSITED, AND THEN SUBJECTING SAID COATED STRUCTURE TO A CATHODIC ELECTROMOTIVE FORCE IN AN AQUEOUS ELECTROLYTE CONTAINING AN ALKALI METAL PYROPHOSPHATE WHICH, THROUGH THE COMBINED ACTION OF THE PYROPHOSPHATE ION AND THE HYDROXYL ION LIBERATED AT THE CATHODE, WILL INCREMENTALLY SOLUBILIZE THE METAL COMPONENT OF THE COATING AS COMPLEX IONS AT THE CATHODE WHILE AT THE SAME TIME THE SELECTED METAL IS ELECTRODEPOSITED ON THE CATHODE FROM THE SOLUBILIZED PRODUCTS. 